1. Field of the Invention
This invention relates to an apparatus that performs annealing by means of a laser beam.
2. Description of the Related Art
When a workpiece such as a semiconductor substrate, for example, is subjected to ion implantation, the disturbing impact of the ions during the implantation process causes the crystalline structure of the surface to undergo a transformation to an amorphous state. Following such ion implantation, therefore, annealing is applied to recrystallize the surface. This annealing is performed by irradiating the surface with a laser beam, with the annealing effect being provided by the optical energy of the laser beam being absorbed as heat energy.
If the laser beam used to irradiate the semiconductor substrate has a low energy level, annealing will not take place, while a laser beam that has too high an energy level will cause ablation of the substrate surface, also thereby preventing the annealing from taking place. For the annealing, therefore, the intensity of the laser beam has to be set at an optimum level.
In conventional systems this optimization of the intensity of the laser beam is achieved in a number of ways. In accordance with a first method, for example, the intensity of the laser beam is controlled by controlling the input to the laser oscillator that generates the laser beam. In a second method, the intensity of the irradiating laser beam is controlled by being passed through a filter that has been added to the optical system. In a third method in which annealing is performed by subjecting the semiconductor substrate to large numbers of low energy laser pulses, the irradiating intensity of the laser beam on the semiconductor substrate is controlled by controlling the number of pulses.
However, in the case of the first method, when there is a wide variation in the range of the controlled input to the laser oscillator, the characteristics of the laser oscillator itself are affected, leading to unstable operation. In the case of the second method, the laser beam intensity that is obtained is determined by the filter used, but continuous control of the irradiation intensity of the laser beam is not always possible. Moreover, when the laser has a short wavelength, such as the 157 nm wavelength of an F.sub.2 laser, degradation of the filter can change the transmission factor. While it may be possible to obtain a filter that maintains its performance over an extended period, such filters are extremely costly. In the case of the third method, when the semiconductor substrate is annealed by irradiating it with a large number of laser pulses, the increased length of time thereby required for the annealing process means a corresponding decrease in productivity.
Thus, in carrying out the annealing process in accordance with the prior art, it is not always possible to provide reliable, continuous control of the intensity of the annealing laser beam.